Silver pastes, such as a resin-type silver paste and a baked-type silver paste, have been frequently used for forming a wiring layer, an electrode, and the like of electronic devices. The heat-curing or heat-baking of these silver pastes after applying or printing thereof allow an electrically conductive film of a wiring layer, an electrode, or the like to be formed.
For example, a resin type silver paste comprises a silver powder, a resin, a curing agent, a solvent, and the like, and this resin type silver paste is printed on a conductor circuit pattern or a terminal, and then heat-cured at a temperature of 100 to 200 degrees C. to be made into an electrically conductive film, whereby wiring, an electrode, or the like is formed. On the other hand, a baked type silver paste comprises a silver powder, glass, a solvent, and the like, and this baked type silver paste is printed on a conductor circuit pattern or a terminal, and then heat-baked at a temperature of 600 to 800 degrees C. to be made into an electrically conductive film, whereby wiring, an electrode, or the like is formed. In the wiring or the electrode formed of these silver pastes, there is formed an electric current path having an electrical connection made by silver powders being ranged.
A silver powder used for silver pastes has a particle diameter of approximately 0.1 μm to several μm, and the particle diameter is dependent on the thickness of a wire or an electrode to be formed. Furthermore, a silver powder uniformly dispersing in the paste allows a wire having a uniform thickness or an electrode having a uniform thickness to be formed.
Characteristics required of a silver powder for silver pastes are different, depending on uses and conditions for use, but, general and important characteristics of a silver powder are a uniform particle diameter, less aggregation of particles, and high dispersibility into the paste. This is because a uniform particle diameter and high dispersibility into the paste allows the paste to be uniformly cured or baked, whereby an electrically conductive film having a low resistance and a high strength can be formed. On the contrary, a nonuniform particle diameter and poor dispersibility cause silver particles not to be uniformly present in a printed film, and accordingly leads to not only nonuniformity in thickness of a wire or an electrode but also unevenness of curing or baking, and therefore causes higher resistance of an electrically conductive film or a fragile and weak electrically conductive film to be.
Furthermore, as a characteristic required of a silver powder for silver pastes, it is also important that the silver powder can be manufactured at low cost. This is because the silver powder is a main ingredient of the paste and accordingly accounts for a large share of a paste price. In order to reduce a manufacturing cost, it is important not only high productivity and a low unit price of raw materials or materials to be used, but also low treatment-costs of waste fluid and exhaust gas.
In many cases, the above-mentioned silver powder to be used for silver pastes has been manufactured by a batch method in which a reductant solution is fed into a tank containing an amine complex of a silver salt, such as silver nitrate, thereby being reduced. However, in the batch method, a reduction reaction starts locally at a point where a reductant solution is fed in, and then nuclei of silver particles are formed at all times from the start of feeding the reductant solution to the completion thereof, and therefore it is difficult to obtain a silver powder having a uniform particle diameter.
A proposal to improve particle size distribution is made also for the silver powder production method in which reduction by the batch method is applied. For example, Patent Literature 1 discloses a method for producing a silver powder, wherein a slurry containing an amine complex of a silver salt and an amine complex of a heavy metal salt which functions as a habit modifier at the time of a reduction reaction is mixed with a solution containing potassium sulfite as a reductant and gum arabic as a protective colloid, thereby reducing the amine complex of the silver salt, and collecting formed silver particles.
According to this production method, there can be obtained a particulate silver powder which comprises primary particles having an average particle diameter of 0.1 to 1 μm and has less particle-aggregation and a narrow particle size distribution. However, according to this production method, a silver salt is reduced under the presence of an amine complex of a heavy metal, and accordingly the heavy metal is easily mixed in as an impurity, and therefore there is a possibility that a silver powder obtained has a low purity. Furthermore, Patent Literature 1 does not disclose a specific particle size distribution, and accordingly it is not clear how particle size distribution the silver powder has.
On the other hand, there has been made an another proposal to improve a particle size distribution by applying a continuous method in which a solution containing an amine complex of a silver salt and a reductant solution are continuously mixed and reduced. For example, Patent Literature 2 discloses a method for producing a silver powder, wherein a silver amine complex solution S1 is flown through a certain first flow path a; and an organic reductant and an additive S2 as needed are flown through a second flow path b which is installed so as to join the first flow path a at a midpoint in the first flow path; and then the silver amine complex solution S1, the organic reductant, and the additive S2 come into contact and are mixed at a junction m of the first flow path a and the second flow path b, whereby reduction and precipitation are performed.
However, a silver powder obtained by this method is in the form of minute particles having an average particle diameter DIA of primary particles of not more than 0.6 μm and a crystallite diameter of not more than 10 nm, each being measured by image analysis of a scanning electron microscope image, and therefore is not suitable for the application to common silver pastes, and the use of the silver powder is limited. Furthermore, the silver concentration in a reaction solution is low, and therefore it is hard to say that this production method is excellent in productivity.
Here, including the cases of the above-mentioned prior production methods, silver nitrate has been commonly used as a silver source raw material. However, in a process to dissolve silver nitrate in ammonia water or the like, toxic nitrous acid gas is generated, and accordingly an apparatus to collect the gas is needed. Also, a large amount of nitrate nitrogen and ammonia nitrogen is contained in waste water, and therefore an apparatus to treat these nitrogen is also needed. Furthermore, silver nitrate is a hazardous substance and also a deleterious substance, and therefore needs to be carefully handled. As mentioned above, in the case where silver nitrate is used as a raw material of a silver powder, there is a problem that silver nitrate has a larger impact and risk on environment than other silver compounds do.
Therefore, there has been proposed a method for producing a silver powder by reducing silver chloride without using silver nitrate as a raw material. The advantages of silver chloride are that silver chloride is neither a hazardous substance nor a deleterious substance, and is a silver compound which can be relatively easily handled although it needs to be shielded from light. Furthermore, silver chloride also serves as an intermediate product in a silver refinery process, and there has been offered silver chloride having a purity sufficient for the use in electronics industry.
For example, Patent Literature 3 discloses a method for producing a silver powder, wherein silver chloride is dissolved in ammonia water so as to obtain a solution having a silver concentration of 1 to 100 g/1, and then a reductant is added to this solution and stirred under the presence of a protective colloid, and liquid phase reduction of a silver amine complex contained in the solution is carried out, whereby ultrafine silver particles are obtained. However, since the silver powder obtained by this method has a very small particle diameter, that is, a particle diameter of not more than 0.1 μm, the use thereof in electronics industry has been limited.
As mentioned above, many methods for producing a silver powder have been ever proposed, but, there has never been provided a method which realizes both producing a silver powder having a narrow particle size distribution, namely, having an average particle diameter of 0.1 μm to several μm and a uniform particle diameter, and producing a silver powder with excellent productivity and at low cost.